Photolithography is a major process for the fabrication of integrated circuits (ICs). A photolithography process transfers patterns on a reticle to a photoresist layer formed on a substrate.
The photolithography process is usually performed by a photolithography apparatus. The photolithography apparatus includes a wafer stage for loading a substrate; a reticle stage, disposed above the wafer stage, for loading a reticle; a light source, disposed above the reticle, for providing the exposure light; and an optical projection unit, disposed between the reticle stage and the wafer stage, for projecting light passing through the reticle onto the substrate.
During the exposure process, an overlay measurement is required to determine if the current layer is aligned with the previous layer over the substrate. Such an overlay measurement is able to ensure the alignment between patterns formed on the current layer and patterns formed on the previous layer.
The existing overlay measurements include the image-based overlay (IBO) technology and the diffraction-based overlay (DBO) technology. Because the bright-field detection is easily affected by various defects on a substrate, such as the rough background on the substrate, and the deformation of the overlay marker during a chemical mechanical planarization process, etc., the IBO technology is already unable to meet the overlay measurement requirements for new technical nodes. Thus, the DBO technology is becoming one of the primary means of the overlay measurement.
The DBO technology acquires overlay errors by measuring the asymmetry of the light intensity between the positive and the negative diffraction orders in the angular resolution of the diffracted light from overlay markers. Although the DBO technology is able to determine the existence of overlay offset, i.e., whether there is an offset, during the overlay measurement, it is difficult to obtain the specific value of the overlay offset. The disclosed methods and systems are directed to solve one or more problems set forth above and other problems.